The phenomenon of superconductivity has not lost its fascination ever since its discovery in 1911. The flow of electric current without friction amounts to the realization of the old human dream of a perpetuum mobile. The ratio of resistance between the normal-conducting and the superconducting (SC) state has been tested to exceed 1014, i. e., it is at least as large as between a usual insulator and copper as the best normal-conducting material .
But superconductivity is more than just the disappearance of resistance: The Meißner effect, the expulsion of magnetic fields from a superconductor, discovered in 1933, shows that superconductivity is a true thermodynamical state of matter since, in contrast to the situation for a merely perfect conductor, the expulsion is independent of the experimental history . As the progress of cooling technique gave access to lower and lower temperatures, superconductivity established as common low-temperature instability of most, possibly all metallic systems (see Fig. 1). As the apparent T→ 0 K state of metals, the zero-entropy postulate of thermodynamics for this limit points to its nature as a macroscopic quantum state.